If import-dsc is not run from within a Git repository it tries to create
one named like the souce package. If that directory already exists we
get confusing message like:
gbp:info: No git repository found, creating one.
gbp:warning: Version 0.7.0 already imported.
gbp:info: Everything imported under /foo/git-buildpackage
Avoid this by checking if the directory exists and aborting in this
case.
Closes: #766350